This invention relates to data communications equipment (e.g. modems) and is particularly useful in a multi-point arrangements in which a central station communicates with a number of remote stations sequentially.
It is well known in the art that in establishing communications between modems, a training sequence of fixed duration may be sent between the data communications equipment (modem) sending the information to the data communication equipment (modem) receiving the information prior to the sending of actual data. The training sequence is typically used to synchronize the timing between the modems and to permit the receiving modem to compensate for the amplitude and phase delay characteristics of the channel over which the data is to be sent. The amplitude and phase delay characteristics which create intersymbol interference are often corrected through the use of equalizers or adaptive equalizers located in the receiving modem. Examples of such equalizers and techniques relating to the same may be seen with reference to the following U.S. patents which are evidence of the state of the art: U.S. Pat. Nos. 4,606,044 to Kudo; 4,571,732 to Pirani et al.; 4,375,692 to Shensa et al.; 4,343,041 to Forney, Jr.; 4,100,495 to Luvison et al.; 4,061,978 to Motley et al.; 4,006,352 to Sato; 4,004,226 to Qureshi et al.; 3,978,407 to Forney, Jr. et al.; 3,715,666 to Mueller et al.; 3,660,761 to Harmon, Jr. et al.; and 3,479,458 to Lord, et al. Reference may also be had to Qureshi, Shahid "Adaptive Equalization", IEEE Communications Magazine, (March 1982, pp. 9-16), and Qureshi, Shahid "Adaptive Equalization", Proceedings of the IEEE, Vol. 73, No. 9 (September 1985 pp. 1349-1387).
As will be appreciated by those skilled in the art, the durations of training sequences have been fixed to permit modems to adapt to the worst case line scenarios. Because the accuracy of the equalization becomes more critical as the data rate of the modem increases, it is not uncommon to find training sequences (RTS-CTS delay) of between 0.25 to 1.5 seconds in length for modems operating at or greater than 9600 bits per seconds (bps). Of course, lengthy training times have been viewed unfavorably as they increase "overhead", and it has been a desire in the art to provide equalization techniques which can reduce the training time required.